The present invention relates to method and apparatus for packaging articles and more particularly to method and apparatus for automatically covering and sealing articles with an extruded sheet of thermosetting or thermoplastic resin material.
The desirability of automated high speed equipment for packaging articles of all types and shapes is apparent. Moreover, packaging which provides a see-through film or sheet covering the article is all the more desirable as it affords visual display of the consumer article while protecting the contents from abrasion, exposure to the air, infection with germs, etc. It is a common practice today in th food industry to package meat products and other foodstuffs in container trays followed by a wrapping of clear plastic sheet or film material.
One particularly advantageous technique for providing such packages has been developed and is disclosed in my U.S. Pat. Nos. 3,008,834; 3,178,866; 3,297,809; 3,284,983; and 3,220,379. As disclosed therein, method and apparatus are provided for extruding and projecting molten film-like sheets of thermoplastic resin material and simultaneously moving the food products or other articles to be packaged along a path which intercepts the projected resin sheet causing the articles to be enveloped thereby. The molten sheet rapidly cools as it is projected from a film forming head such that as the material engages the moving article, it has sufficiently solidified to maintain its film-like continuity. The sheet or film itself is generated by extruding a liquified viscous resin material outwardly from a die or orifice constituting the critical component of the film forming head. This extrusion is performed by applying substantial pressure to the fluid at a point upstream of the extruding head.
In prior systems, such as the one disclosed in U.S. Pat. No. 3,178,866, the extruded and projected plastic sheet is tensioned during the interval between receipt of the articles to be packaged by projecting the sheet downwardly from a film forming head overlying the conveyor and engaging the downwardly depending sheet at a location underlying the path of the conveyed articles with a rotating roller surface moving at a speed slightly greater than the rate at which the film is extruded. While this arrangement has been found suitable for some packaging operations, it only provides tensioning of the film at the time that the leading edge of the article intercepts the sheet. Once this interception occurs, the moving article redirects the downwardly depending sheet, such that the tensioning effect of the roller is disrupted. Coverage of portions of the article to the rear of the leading edge depend upon the gravitational forces operating on the extruded film. Such forces are not substantial enough to snug the material down and seal it to the article.
Furthermore, wrapping of the articles in this manner results in tails or trailing pieces of film which must be removed in a separate operation to complete the package. Also this technique does not afford wrapping of the sides of such articles.